1. Field of the Invention
The field of the invention relates to medical devices, and more particularly, to systems and methods for ablating or otherwise treating tissue using electrical energy.
2. Background of the Invention
Tissue may be destroyed, ablated, or otherwise treated using thermal energy during various therapeutic procedures. Many forms of thermal energy may be imparted to tissue, such as radio frequency electrical energy, microwave electromagnetic energy, laser energy, acoustic energy, or thermal conduction.
In particular, radio frequency ablation (RFA) may be used to treat patients with tissue anomalies, such as liver anomalies and many primary cancers, such as cancers of the stomach, bowel, pancreas, kidney and lung. RFA treatment involves the destroying undesirable cells by generating heat through agitation caused by the application of alternating electrical current (radio frequency energy) through the tissue.
Various RF ablation devices have been suggested for this purpose. For example, U.S. Pat. No. 5,855,576 describes an ablation apparatus that includes a plurality of wire electrodes deployable from a cannula or catheter. Each of the wires includes a proximal end that is coupled to a generator, and a distal end that may project from a distal end of the cannula. The wires are arranged in an array with the distal ends located generally radially and uniformly spaced apart from the catheter distal end. The wires may be energized in a monopolar or bipolar configuration to heat and necrose tissue within a precisely defined volumetric region of target tissue. The current may flow between closely spaced wire electrodes (bipolar mode) or between one or more wire electrodes and a larger, common electrode (monopolar mode) located remotely from the tissue to be heated. To assure that the target tissue is adequately treated and/or to limit damaging adjacent healthy tissues, the array of wires may be arranged uniformly, e.g., substantially evenly and symmetrically spaced-apart so that heat is generated uniformly within the desired target tissue volume. Such devices may be used either in open surgical settings, in laparoscopic procedures, and/or in percutaneous interventions.
During tissue ablation, the maximum heating often occurs in the tissue immediately adjacent the emitting electrodes. In general, the level of tissue heating is proportional to the square of the electrical current density, and the electrical current density in tissue generally falls rapidly with increasing distance from the electrode. The decrease of a current density depends upon a geometry of the electrode. For example, if the electrode has a spherical shape, the current density will generally fall as the second power of distance from the electrode. On the other hand, if the electrode has an elongate shape (e.g., a wire), the current density will generally fall with distance from the electrode, and the associated power will fall as the second power of distance from the electrode. For the case of spherical electrode, the heating in tissue generally falls as the fourth power of distance from the electrode, and the resulting tissue temperature therefore decreases rapidly as the distance from the electrode increases. This causes a lesion to form first around the electrodes, and then to expand into tissue disposed further away from the electrodes.
Due to physical changes within the tissue during the ablation process, the size of the lesion created may be limited. For example, the concentration of heat adjacent to wires often causes the local tissue to desiccate, thereby reducing its electrical conductivity. As the tissue conductivity decreases, the impedance to current passing from the electrode to the tissue increases so that more voltage must be supplied to the electrodes to affect the surrounding, more distant tissue. The tissue temperature proximate to the electrode may approach 100° C., so that water within the tissue boils to become water vapor. As this desiccation and/or vaporization process continues, the impedance of the local tissue may rise to the point where a therapeutic level of current can no longer pass through the local tissue into the surrounding tissue.
Thus, the rapid fall-off in current density may limit the volume of tissue that can be treated by the wire electrodes. As such, depending upon the rate of heating and the size of the wire electrodes, existing ablation devices may not be able to create lesions that are relatively large in size. Longer wire electrodes and/or larger arrays have been suggested for creating larger lesions. The effectiveness of such devices, however, may be limited by the desiccation and/or vaporization process discussed previously. While wire electrodes can be deployed, activated, retracted, and repositioned sequentially to treat multiple locations within a tissue region, such an approach may increase the length of time of a procedure, and precise positioning to ensure that an entire tissue region is treated may be difficult to accomplish.
Accordingly, improved systems and methods for tissue ablation would be useful.